The present invention is directed towards a superabsorbent polymer, particulate superabsorbent polymer compositions, methods to make such products and absorbent articles containing such products, and to a process to make a crosslinker composition. Examples of superabsorbent polymer may include a crosslinked partially neutralized polymer, including crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, that are capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining the aqueous liquids under a certain pressure in accordance with the general definition of superabsorbent polymer. Superabsorbent polymer may be formed into particles, generally referred to as particulate superabsorbent polymer, wherein the particulate superabsorbent polymer may be post-treated with surface crosslinking, surface treatment, and other treatment to form particulate superabsorbent polymer compositions. The acronym SAP may be used in place of superabsorbent polymer, superabsorbent polymer composition, and particles hereof. A primary use of superabsorbent polymer and superabsorbent polymer compositions is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCR, New York, 1998.
Superabsorbent polymers may be prepared by initially neutralizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of a caustic treatment, such as sodium hydroxide, and then polymerizing the product with a relatively small amount of an internal, or monomer, crosslinker such as a di- or poly-functional monomer. The di- or poly-functional monomer materials may serve as covalent internal crosslinking agents to lightly crosslink the polymer chains, thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked superabsorbent polymers contain a multiplicity of carboxyl groups attached to the polymer backbone. These carboxyl groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network.
In addition to covalent internal crosslinking agents, ionic internal crosslinking agents have been utilized to prepare superabsorbent polymers. The ionic internal crosslinking agents are generally coordination compounds comprising polyvalent metal cations, such as Al3+ and Ca2+, as disclosed in U.S. Pat. No. 6,716,929 and U.S. Pat. No. 7,285,614. The superabsorbent polymers disclosed in these patents have a slow rate of absorption, due to the presence of ionic crosslinks. In this context, the absorption rate may be measured by a Vortex Test.
Superabsorbent polymers, useful as absorbents in absorbent articles such as disposable diapers, need to have adequately high absorption capacity, as well as adequately high gel strength. Absorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength relates to the tendency of the swollen polymer particles to resist deformation under an applied stress, and needs to be such that the particles do not deform under pressure, and fill the capillary void spaces in the absorbent member, or article, to an unacceptable degree, which is generally called “gel blocking”, thereby inhibiting the rate of fluid uptake, or the fluid distribution, by the member or article. Once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article, and leakage from the absorbent article can take place well before the particles of absorbent polymer in the absorbent article are fully saturated, or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article.
U.S. Pat. No. 6,087,450 is directed to providing internal, or monomer, crosslinking agents and superabsorbent polymers cross-linked with them, as well as a process for making the superabsorbent polymer including such internal crosslinking agents. Such internal crosslinking agents are characterized by the fact that by means of reacting, for example, a saturated glycidyl compound with unsaturated amines, for example, allylamines, they open the epoxide ring, thereby forming a hydroxyl group which is optionally available for a subsequent ethoxylation. There are also other reaction paths to produce the cross-linking agents according to the present invention; for example, reacting amines with unsaturated glycidyl compounds, such as (meth)allyl glycidyl ethers or glycidyl (meth)acrylates. However, it has been found that such polymers made in accordance with the technology of the '450 patent may include high residual amounts of the respective glycidyl compounds, which need to be reduced. In particular, it has been found that such polymers may include about 2500 ppm of the respective residual glycidyl compounds. Reduction of the amount of residual glycidyl in the internal crosslinker would make this crosslinker more suitable for the manufacture of polymers such as superabsorbent polymers.
Therefore, there is still a need to improve the content of the respective glycidyl compound(s) of the superabsorbent polymers as set forth in the '450 patent by reducing the residual glycidyl compound(s).